The present disclosure relates to a system and a method for preventing powder depletion/contamination during a consolidation process.
To make a superalloy billet utilizing the powder metallurgy route, a powder of given composition is introduced through the opening in an inert container (can), which ultimately will undergo compaction and extrusion at high temperature. The can is typically made out of a higher grade stainless steel tube with lids welded to it. The tube can be either a centrifugally cast tube, or a rolled one from a sheet and welded. The can protects the powder from the environment and, in addition, it acts to provide self-lubrication during the extrusion process.
The primary purpose of using a higher grade stainless steel is to minimize the effect of chemical composition dissimilarity of materials (can and powder). Compositional differences between the can and the powder drive diffusion reactions during the compaction and extrusion steps resulting in a zone of alloy depletion/contamination to a depth of approximately 0.0250″ in the powder. The depleted zone may be further disturbed during the in-process manipulation (handling or thermal shock), it may crack and the crack becomes filled with the depleted particles that further deteriorate the quality of the billet. Such areas will be revealed by a macro-etch method during the quality inspection of the billet—they will appear as etching lines generally aligned with the billet axis and progressing radially inward, therefore they are called linear etch indications (LEI).
Since LEIs have the potential of yielding non-uniform properties they must be removed which results in lower yields of the consolidated billet. Occasionally, LEIs may be undetected in the billet and carried further in the process to be detected only at the final stage of component manufacturing (resulting in scrap). There is a significant financial impact in lower yield and scrap.